1. Field of the Invention (Technical Field)
The present invention relates to offshore drilling installations and methods.
2. Description of Related Art:
A known method and a known offshore drilling installation are known from WO02/36931 and can be illustrated with reference to FIG. 1. As opposed to subsea drilling as known from U.S. Pat. No. 3,741,320, offshore open hole drilling is normally done using surface mounted drilling equipment on a platform, like a (floating) vessel 1 as shown in FIG. 1, or on a pontoon or jack-up (fixed to the seabed via legs).
The drilling equipment of FIG. 1 comprises: means to assemble (make) and disassemble (break) the drill string; a motor 2 to rotate the drill string 7; a winch 3 to lower, lift and feed the drill string 7; and, in case of a floating vessel, a heave motion compensator 4.
In this prior art installation control of the weight on bit (WOB) and the rate of penetration (ROP) of the drill bit 5 are controlled from the platform, with guidance from bit guide 16. The drill string 7 is driven continuously or discontinuously until it reaches target depth. A typical application is for geotechnical site investigation. In offshore drilling for geotechnical site investigation down hole tools are used to determine various parameters of the soil, like strength, type, etc. These tools require a stationary drill bit 5 in relation to the seabed 27 in order to collect good quality data. The surface mounted means to compensate for heave motion 4 are in this respect not ideal and have errors that result in displacement or force variation at the drill bit 5. In normal operations these variations are directly transferred to the drill bit 5 creating soil disturbances under the drill bit 5. Once the drill bit 5 is advanced to a particular depth of interest the rotation is stopped and the drill string 7 is fixed to the seabed template 26 using a stationary clamp 15. During sampling the errors of the heave motion compensator 4 will be transferred into the seabed template 26 and from the template into the seabed 27. Although the template is sized in weight and bearing area that residual motions are reduced to an acceptable level, the industry is still looking for improvement.
The clamping means mounted in the seabed template according to WO02/36931 are embodied as a chuck to clamp the drill string whilst still enabling the drill string's rotation. Various chuck designs exist that allow free rotation while vertically constraining the pipe. This enables controlling the drill pipe at the lower end (at seabed), which has a major advantage over control at the top as displacement errors and force variations errors at the drill bit are much less resulting in a better controlled drilling process. Other prior art offshore drilling installations with surface mounted drilling equipment only clamp the drill string when sampling is done and the pipe is not rotating.